Always show subscribed newsletters on /existing

Short version: on /existing, don't worry about what languages a
newsletter supports if the user is already subscribed to it.
Always show it, and don't complain if the form language is changed
to one that the newsletter doesn't support, the subscribed box
is left checked, and the form is submitted.  Let the user continue
to subscribe to it.

Also made some fixes to the javascript that hides and displays the
newsletters on the page. It wasn't really working at all.

Bug 857271
Merge branch 'bug-845904-newsletter-preference-center' into bug-857271-newsletter-languages

Also remove the javascript from existing.html that I was
about to remove anyway.

Conflicts:
	apps/newsletter/templates/newsletter/existing.html
Be more flexible about languages

Changes after our call today:
- Don't care about language when deciding which newsletters to show
- Don't care about language when deciding whether to let them subscribe
  to a newsletter
- Display "English only" on newsletters that are.

Bug 857271
Merge branch 'dpoirier-bug-857271-newsletter-languages' into bug-845904-newsletter-preference-center

Conflicts:
	apps/newsletter/templates/newsletter/existing.html